This class also indirectly enables syntax such as the following regular test registration
in FlatSpec and org.scalatest.fixture.FlatSpec:

"A Stack (when empty)" should "be empty" in { ... }
^

However, this class does not declare any methods named in, because the
type passed to in differs in a FlatSpec and a org.scalatest.fixture.FlatSpec.
A org.scalatest.fixture.FlatSpec needs two in methods, one that takes a no-arg
test function and another that takes a one-arg test function (a test that takes a
Fixture as its parameter). By constrast, a FlatSpec needs
only one in method that takes a by-name parameter. As a result,
FlatSpec and org.scalatest.fixture.FlatSpec each provide an implicit conversion
from ResultOfStringPassedToVerb to a type that provides the appropriate
in methods.